1. Field of the Invention
This invention relates to an apparatus and method for computing the position of a mobile device by use of wireless signals, such as GPS systems.
2. Description of Related Art
Existing position location technologies based on GPS use a network of satellites that transmit signals at a predefined GPS frequency at a known time. A GPS receiver on the ground measures the times of arrival of the signals from each satellite in the sky it can xe2x80x9cseexe2x80x9d. The times of arrival of the signals along with the exact location of the satellites and the exact times the signals were transmitted from each satellite are used to triangulate the position of the GPS receiver. A standard GPS receiver includes a local oscillator that is used to receive the GPS signals from the satellites.
The acquisition of signals from the GPS satellites is highly sensitive to frequency variations in the local oscillator of the GPS receiver. A number of factors contribute to making the acquisition of the GPS signals difficult. The GPS signals are transmitted at a relatively low power, and the GPS satellites are in earth orbit. By the time the GPS signals reach the ground, their initially low power has been greatly reduced, making the signal extremely weak. As a result, if the GPS receiver""s local oscillator frequency is even slighty off the GPS frequency, it may be difficult and time consuming to effectively receive the GPS signals.
In many communication systems, including GPS receiving systems, there exists a primary local oscillator, called a xe2x80x9creference local oscillatorxe2x80x9d. The output signal of the reference local oscillator is in turn fed to one or more frequency synthesis circuits, which in turn produce additional signals at other frequencies that are provided to various circuits in such systems. As an example, it is common for GPS receiving systems to utilize a reference local oscillator with a nominal output frequency of 16.368 MHz. This oscillator output is typically fed to a frequency synthesis circuit that uses the reference frequency to produce a local oscillator frequency in the vicinity of 1575.42 MHz, which in turn is used in a downconverter circuit that translates the input GPS signals to near baseband. Similarly this reference oscillator output may be used to synthesize a sample clock, often with a frequency being a multiple of 1.023 MHz, where this sample clock is used as part of a digitizing circuit that samples the near-baseband translated GPS signal. Similarly, in cellphone receivers, a reference oscillator often has a frequency of oscillation in the 10 MHz to 20 MHz range (depending upon design), which is used to produce additional frequencies for purposes of signal down conversion and sampling.
In some situations it may be advantageous for a GPS system to utilize a frequency calibration method such as disclosed in U.S. Pat. Nos. 5,841,396, 6,421,002, and others. In one approach, the average frequency of the cell phone""s local oscillator (VCTCXO) is measured, and used to calibrate the frequency error of the GPS receiver""s oscillator. This VCTCXO is typically frequency locked to the highly stable received cellular signal. An alternative approach is to frequency lock the GPS oscillator to the cell phone""s local oscillator. This latter approach suffers in performance if the short term stability of the cell phone""s oscillator is poor. Such cell phone oscillator stability is dependent upon a number of factors, such as the cell phone design, and platform motion.
The calibration method sometimes suffers performance problems, including:
(A) frequency xe2x80x9cracingxe2x80x9d problems due to heating/cooling associated with the cell phone""s transmitter turning on/off;
(B) frequency variation due to voltage fluctuations, again associated with the cell phone""s transmitter turning on/off; and
(C) other heating/cooling situations due to environmental effects.
Although the above calibration measurement method may work adequately in a platform that is static or moving at relatively slow speeds, such as pedestrian speeds, rapid frequency fluctuations of the cellular oscillator observed in vehicular usage will cause the performance of the calibration to deteriorate. That is, because the cellular oscillator is frequency locked to the network, rapid fluctuations in received frequency of the cellular signal may cause large errors in calibration. Such rapid fluctuations may occur, for example, when a vehicle is approaching a serving cell base station and then passes the station. During this short time, the observed frequency of the direct pilot from this cell base station may rapidly change from a large positive Doppler (e.g. +100 Hz) to a large negative Doppler (e.g. xe2x88x92100 Hz) in a matter of a second or several seconds. This rapid variation in received cellular carrier frequency, coupled with a cell phone transmitter being turned on or off, may cause a deterioration of the efficacy of the frequency-locked calibration method.
To provide accurate and quick position measurements in a practical mobile position location device, a system described herein calibrates a GPS receiver by predicting a frequency error in the next time period responsive to a first frequency locked to an externally transmitted signal and a second frequency generated by a GPS oscillator. Particularly, the calibration system makes several measurements over time, estimates an error in each measurement, approximates an error function, and predicts an error for the next time period. The predicted error is then used to correct the GPS receiver in the next time period.
A method and apparatus is disclosed for calibrating and correcting a GPS receiver in the mobile device using an externally transmitted signal (other than a GPS signal) that has a predefined precision carrier frequency. To receive the GPS signal transmitted at a predefined GPS frequency, the method includes generating a first frequency signal responsive to the precision carrier frequency, and generating a second frequency signal in the GPS receiver that is applied to process the GPS signal. For example, the first frequency may be a subharmonic of the frequency received by a cellular receiver, and the second frequency signal may be generated, directly by a reference GPS oscillator in the GPS receiver, or it may be derived from a reference GPS oscillator. The calibration method includes estimating an error between the first and second frequencies in a first time period, repeating the error estimating step for at least one additional time period to provide a set of error estimations, approximating an error function of the second frequency responsive to the set of error estimations, and predicting an error in a next time period utilizing the approximated function. A correction signal is generated, and an oscillator in the GPS receiver is corrected in the next time period to process the GPS signal responsive to the predicted error.
Typically, the calibration method includes repeatedly measuring a ratio of the second and the first frequencies over a plurality of time periods, and for each time period comparing this ratio to a predetermined number to estimate the error respectively for each time period. The error prediction step typically includes determining a frequency error vs. time of the second frequency by fitting a mathematical function responsive to the set of error estimations, such as by averaging the set of error estimations, and performing a mathematical regression method utilizing the set of error estimations to produce a least-mean-square fit to the set of error estimations.
In some disclosed methods, the correcting step includes correcting a GPS local oscillator that supplies the second frequency to convert the GPS signal at the GPS frequency into a predetermined intermediate frequency, and in other disclosed methods the GPS receiver comprises a digital processing system that includes a digital local oscillator, and the correcting step includes correcting the digital local oscillator.
In one implementation, a multiple function portable device is disclosed for providing cellular communication using a network of cellular stations that operate at predefined ideal cellular frequencies, and also for providing position location using GPS satellites that transmit GPS signals at a predefined GPS frequency. The multi-function portable device comprises a cellular communication system including a first local oscillator that generates a first frequency for demodulating communications with the network of cellular stations, a GPS system including a second local oscillator that generates a second frequency coupled to process the GPS signals, an error prediction and correction system that predicts an error in a next time period responsive to measurements of the first and second frequencies over a plurality of previous time periods, the system also generating a correction signal for the next time period responsive to the predicted error, and a local oscillator in the GPS system that receives the correction signal and responsive thereto processes the GPS signal during the next time period.